In my prior U.S. Pat. No. 3,585,527 I have disclosed a system of this character wherein oscillations of a quartz crystal are sustained by an energizing circuit including a pair of field-effect transistors of the MOS/FET type, the first MOS/FET having an insulated gate connected to a terminal of the crystal whereas the insulated gate of the second MOS/FET is fixedly biased by a resistive voltage divider bridged across a supply battery. A biasing resistor for the gate of the first MOS/FET is connected across the battery in series with a capacitive voltage divider shunting the crystal, a tap of the latter divider being tied to a junction between the source of the first and the drain of the second MOS/FET. Only the first transistor constitutes an electronic switch serving as an active element of what is essentially a Colpitts oscillator; the second transistor only controls the current supplied to the switching MOS/FET.
The continuously conducting second transistor of my prior system consumes about half the energy supplied by the battery, the power dissipated therein being given by the product of its mean drain voltage and its current. The overall efficiency of that system is on the order of 0.3, with an upper limit of about 0.4.